S&W will not convert a 3rd Gen to a DAO. They will convert a Value Line or CS Series to DAO, since the mechanism changed.
The 3rd Gen DAO requires a different frame and slide, since the hammer is cocked half way by the action of the slide.
On the Value Line and CS Series autos, they utilized the same DA/SA frame and slide, and made it so the slid is cocked only a quarter of the way by the action of the slide.
If you want a DAO, trade your DA/SA for a DAO version of the same pistol.
What Gen 3 are you interested in converting.